Permanent magnets based on compositions containing iron, neodymium and/or praseodymium, and boron are known and in commercial usage. Such permanent magnets contain as an essential magnetic phase grains of tetragonal crystals in which the proportions of, for example, iron, neodymium and boron are exemplified by the empirical formula Nd.sub.2 Fe.sub.14 B. These magnet compositions and methods for making them are described by Croat in U.S. Pat. No. 4,802,931 issued Feb. 7, 1989. The grains of the magnetic phase are surrounded by a second phase that is typically rare earth-rich, as an example neodymium-rich, as compared with the essential magnetic phase. It is known that magnets based on such compositions may be prepared by rapidly solidifying, such as by melt spinning, a melt of the composition to produce fine grained, magnetically isotropic platelets of ribbon-like fragments. Magnets may be formed from these isotropic particles by practices which are known, such as bonding the particles together with a suitable resin.
Although the magnets formed from these isotropic ribbons are satisfactory for some applications, they typically exhibit an energy product (BHmax) of about 8 to about 10 megaGaussOersteds (MGOe), which is insufficient for many other applications. To improve the energy product, it is known to hot press the isotropic particles to form magnets having an energy product of about 13 to about 14 MGOe. Lee, U.S. Pat. No. 4,782,367, issued Dec. 20, 1988, went on to demonstrate that the melt-spun isotropic powder can be suitably hot pressed and hot worked by plastically deforming to create high strength, magnetically anisotropic permanent magnets. Being magnetically anisotropic, such magnets exhibit excellent magnetic properties, typically having an energy product of about 28 MGOe or higher. However, a shortcoming of the anisotropic magnets is that, because the final forming step is a plastic deformation process, the shapes in which the anisotropic magnets can be formed are significantly limited, particularly in comparison to the great variety of shapes which are possible with bonded and hot pressed isotropic magnets.
Another shortcoming with the production of anisotropic magnets is that the several processing steps required are time consuming, and the added hot working step increases the costs for making these magnets. In addition, the dies and punches required to hot work the magnets are generally complicated. As a result, anisotropic permanent magnets are typically more expensive to produce and, again, their shapes are limited by the equipment required to form them.
Magnets composed of bonded anisotropic particles having an energy product of about 15 to about 18 MGOe are known. The anisotropic particles are formed from hot-worked, anisotropic magnets, such as those described above, by known methods, such as mechanical grinding, pulverization and hydrogen decrepitation methods. The anisotropic particles are then bonded together with a suitable binder, such as a thermoset or thermoplastic, to form a permanent magnet. However, to achieve these high energy product values, it is necessary to subject the particles to an alignment field during processing. As a result, the possible shapes for the permanent magnet are again limited. In addition, processing is more difficult and complicated because the particles are already magnetized, which can be particularly detrimental in the computer industry where stray magnetic particles can seriously damage the operation of memory.
Therefore, although the above prior art permanent magnets are suitable for many applications, it would be desirable to provide a method for forming permanent magnets exhibiting an energy product of at least about 15 MGOe and above, and preferably about 20 MGOe or greater, in which the method has the advantage of being capable of forming permanent magnets having a great variety of shapes and yet does not require either a hot working step or magnetic alignment during hot pressing.